Gliders in general are a version of autonomous underwater vehicles (AUVs), but with one important distinction: they are buoyancy-driven, rather than using a propeller to generate thrust underwater. Gliders maintain the torpedo-like shape of traditional AUVs but typically have wings that provide an extra control surface. They use a variable buoyancy system to change how much they weigh in the water, allowing them to ascend or descend, and also modify weight distribution inside their bodies to change their pitch angle in the water.

For example, the Spray Glider we build at Bluefin (developed at the SIO) has a constant-volume pressure vessel filled with mineral oil that can be pumped into or out of a bladder that sits in a flooded section of the vehicle. When oil is pumped in, the bladder expands, and displaces water in that flooded section, increasing the vehicle's volume in the water and allowing it to ascend. Similarly, when it pumps the oil out of the bladder and back into the constant volume vessel, it permits water to fill the flooded section again, decreasing the vehicle's volume and causing it to sink. Internal battery packs mounted on tracks can move side to side (controlling the vehicle's roll) and forward and backward (letting it dive or climb), so combining the angle control with the varied buoyancy allows the vehicle to dive and climb in a sawtooth pattern through the water. With this sort of system, a glider can stay out for durations on the order of six months.

This technology is ideal for long-term missions. Because so little power is used compared to a propeller that is always spinning, a glider can stay out for months at a time on one battery charge rather than the tens of hours that a propelled vehicle can achieve on a single charge. Webb, however, has made a few modifications to their buoyancy system that may allow their new gliders to stay out for years.

Rather than relying on a battery-powered hydraulic system to pump mineral oil in and out of the bladder, the oil is normally contained in a plastic tube surrounded by a kind of wax chosen for its phase change properties. The wax is sensitive to the ocean temperature: at cold temperatures it contracts, but at warmer ocean temperatures it expands. The expansion increases the volume of the wax and squeezes the tube, which in turn squeezes mineral oil out into the rest of the vehicle's variable buoyancy system. This causes the vehicle to sink. When it reaches depth (and therefore a low water temperature), the wax contracts, which allows the oil to flow back into the tube out of the buoyancy system and the vehicle ascends.

So what Webb has done is eliminate the need for batteries to drive a hydraulic pump system. However, they still need batteries to run the onboard computer, sensors, communication system, and roll/pitch control system, meaning that these gliders aren't totally "green" yet. But they're on their way, and other glider researchers are following suit. Webb's recent demonstration -- the activity that generated all the press -- shows that this technology is ready to be out of the lab and into the ocean, so we can probably expect this system to become widespread pretty quickly. It will be interesting to see what other ocean-powered systems can be applied to these gliders to make them truly "green'.